1. Field of the Invention
The present invention is directed to methods of fabricating profiles of varying transmission, more specifically to creating apodized apertures for use in wavefront sensing and the apertures created thereby.
2. Description of Related Art
Hartmann type wavefront sensors measure the spot positions of light diffracted from an array of apertures to determine the shape of an optical wavefront impinging on the aperture array. The original Hartmann sensor used diffraction from hard apertures put into an opaque screen or plate. The demand for high photon efficiency for some applications required the screen be replaced with an array of lenses, forming a Shack-Hartmann wavefront sensor. The advent of micro-optics allowed small high quality arrays to be fabricated.
Currently, Hartmann-type sensors are used for optical meteorology and laser characterization. Lenses for the Shack-Hartmann wavefront sensor have fairly long focal lengths, since this improves the sensitivity of the sensor to phase tilt by increasing the moment arm and spreading the focal spot over many pixels on a CCD which provides better centroid accuracy. These slow, i.e., large f-number, lenses create large diffraction patterns. A diffraction pattern from an individual lens in the detection plane spreads into the area behind neighboring lenses and creates crosstalk. Coherent sources of radiation exacerbate the crosstalk through interference.
Diffraction plays an important role as well in many different types of optical systems. It plays a critical, limiting role in astronomy, for example. With the advent of large, accurate telescopes that are either space-based, or ground-based but using adaptive optics, it is possible to optically resolve planets in orbit around nearby stars. However, these planets would have very little angular separation from the star, and would appear much dimmer. To block the light from the star a small obscuration disk can be placed at an intermediate image plane in the telescope. However, diffraction from the edges of this obscuration would swamp the image of a planet.
Furthermore, diffraction plays a key role in any light propagation or manipulation. As light propagates, either in free space or through a media, both the phase and irradiance distributions affect its state. There are currently a number of means for controlling the phase state of the light. This can be accomplished through a lens, mirror, phase plate or other optical element. In fact, elements can be fabricated to create arbitrary phase states. However, it is currently not possible to control the irradiance state of the light. If both can be controlled together, then the complete E-field of the light has been specified and hence completely arbitrary control of the light at any plane is possible.
It is therefore an object of the present invention to provide a Hartmann wavefront sensor that overcomes one or more of the problems due to the limitations and disadvantages of the related art. More particularly, it is an object of the present invention to reduce crosstalk in the sensor.
It is further an object of the present invention to be control both the phase and irradiance distribution, thereby completely specifying the E-field of the light, allowing completely arbitrary control of the light at any plane.
It is a further object of this invention to provide for a means of masking, reflecting or blocking light so as to obscure some regions without introducing diffraction rings or other features. It is an object to provide softened apertures and masks that minimize or control the effects of diffraction.
It is a further object of this invention to control the irradiance distribution of light in an arbitrary fashion. Combined with existing means for controlling the phase, it is an object of the invention to provide complete control of the E-field of the light.
These and other objects may be realized by applying an apodized irradiance mask to a lens or substrate such that the transmission of the optical element varies with position in a controlled fashion. For some applications, this mask may be a super-gaussian.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.